If they don't have wheels, they lift just enough to break contact or skids on grass they'll actually slide, then start their run to pick up momentum while gaining altitude.
The rotors on a helicopter are different from the propeller on a plane. Rotors are actually a rotary wing and forward (or any direction) momentum is a great help in getting airborne, safely. A vertical ascent is the least stable situation for a helicopter and subject to an array of possible disruptions.
